Delayed trip for hydraulic circuits



Oct. 23, 1934. y H. ERNST ET Ax.

DELAYED TRIP FOR HYDRAULIOCIRCUITS Filed sept. 15. 1930 s sheets-sheet 1Oct. 23, 1934. H. ERNST Er Al. Y 1,978,346

DELAYED TRIP FOR HYDRAULIOCIRCUITS Filed Sept. l5. 1930 3 Sheets--SheeI2 #fw-JU L/Pwm/o Zo El fa jlfeeaze ammi,

Oct. 23, 1934. H. ERNST 'E T AL 1,978,346

` `IEL'AYED TRIP FOR HYDRAULICYCIRCUITS Filed sept. 15. 1930 ssheets-sheet s 85 'g/@p dmv/l j y A .86 u

Patented a. 23, 1934 DELAYED TRIP FOR HYDRAULIC CIRCUITS Hans'Ernst andCharles W. Goodrich, Cincinnati,

Ohio, assignors to The Cincinnati Milling Ma' l chine Company,Cincinnati, Ohio, a corporation of Ohio Application September 15, 1930,Serial No. 481,939

OFFICE 16 Claims.

' This invention relates to hydraulically actuated machine tools andmore `particularly to an improved mechanism for effecting apredetermined pause in the operative cycle thereof.

In thev construction of hydraulically actuated machine tools and moreparticularly in machines having a translatable part, such as a worksupport, suitable valve mechanism is employed for eiecting a reversal inthe direction of flow of the hydraulic medium to eiect a change in thedirection of movement of the translatable part. Such valve mechanismshave been automatically controlled through mechanical means, as well ashydraulic means. Since it is often desirable in machining some types ofWork to effect a pause in the movement of the work with respect to thecutter whereby the surface of the work may be properly finished beforeit recedes from the cutter, it is an object of this invention to providea new and improved mechanism that may be adapted to hydraulic circuitsfor effecting a delayed action thereof and thereby a pause in the move-'ment of the translatable partof the machine.

Another object of this invention is to provide a delay unit for thereversing mechanism of an hydraulically operated machine tool which isactuated directly by the feeding pressure thereby insuring positiveactuation thereof and making possible increased accuracy in setting thetimeof .reversaL A further object of this invention is to provide adelay unit for the control mechanism of an hydraulic circuit which maybe adjusted to vary the time interval that the action of the controlmechanism is postponed.,

Other objects and advantages of the present invention should be readilyapparent by reference to the following specication considered inconjunction with the accompanying drawings illustrative' of oneembodiment thereof but it will be understood that any modifications maybe made in the specic structural detailsihereinafter disclosed withinthe scope of the appended claims without departing from or exceeding thespirit of the invention.

Referring to the drawings in which like reference numerals indicate likeparts:

Figure 1 is a diagrammatic view of a complete hydraulic system showingthe application of the delay unit thereto in conjunction with the partsoperated and controlled thereby.

Figure 2 is a detail view of the trip mechanism for actuating the'pilotvalve.

Figure 3 is a view of the pilot valve in reversed position.

Figure 4 is a detail section on the line 4 4 of Figure 2. l

Figure 5 is a diagrammatic view of a delay unit applied to an hydraulicreciprocating cycle circuit to effect a delayin each reversal of thetable.

Figure 6 is a longitudinal section through the selector valve.

Figure 7 is a longitudinal section through pilot valve.

Figure 8 is a section on the line 8 8 of Figure 6.

Figure 9 is a section on the line 9-9 of Figure 6.

Figure 10 is a section on the line 10--10 of Figure 7.

In the drawings the reference numeral l0 indicates a reciprocatingsupport of a machine tool, such as a milling machine, which forms anexcellent example for the illustration of this invention. Such machineshave a cutting tool'11 for acting upon Work, such as 12, moved by thetable 10 into contact therewith for effecting a machining operationthereupon. Hydraulic means have heretofore been utilized for effectingthe reciprocation of the table and usually comprises an hydraulic motorof the piston and cylinder type in which the cylinder, such as 13, isusually fixed while the piston 14 relatively movable therein is attachedto the table for imparting movement thereto through a piston rod, suchas 15.

Hydraulic pressure is alternately supplied to opposite ends of thecylinder to effect movement of the piston and for this purpose channels,such as 16 and 17, are connected respectively to opposite ends of thecylinder from which they lead ,to a rate and direction determiningmeans, such 'as a selector valve 18. A stop valve 19 is usuallyinterposed between the selector valve and the cylinder to close orlockthe fluid in opposite ends of the cylinder thereby stopping movementof the table and holding it in such position by means of the fluidtrapped in the opposite ends thereof.

In the hydraulic actuation of milling machines it is highly desirableand necessary to give a uniform feed or movement to the Work withrespect to the cutter to insure proper finish as well as an economicalcutting action of the tool. 'I'his result is accomplished by providing aclosed system for the hydraulic medium inl which the medium is forcedinto one end of the cylinder While being withdrawn from the opposite endin measured'amounts at such rate as to effect the desired feed rate. Onemethod of accomplishing this is to provide a variable displacement pump,as shown at 20, having an inlet.21 and an outlet 22 which are connectedto the selector Yvalve by channels 23 and'24. A rapid traverse pump,suchas 25, having an inlet 26 connected with the reservoir 27 and anoutlet 28 leading to the selector valve, is provided for effecting quicktraverse movements of the table and in such cases 5 the return side ofthe cylinder is connected to the discharge line 29 leading directly tothe reservoir located in the base ofthe machine. To maintain the feedingcircuit under constant pressure and replace fluid losses, due to leakageor other reasons, a booster pump, such as 30, is

provided for injecting replacement fluid in the high pressure side ofthe feed circuit.

The selector valve is rotatable to connec either the feed circuit or therapid traverse circuit with the hydraulic motor to change the rate ofmovement of the table from feed to rapid traverse and it islongitudinally shiftable to reverse the ilow in the channels 16 and 17thereby changing the direction of movement of the table.

20 Due to the pressure of the hydraulic medium flowing through theselector valve and the danger of the valve thus becoming slightlycramped it is desirable, in the interest of positive actuation, to poweroperate the valve. To this end a bracket, such as 31, Figure 8, isprovided on one end 80 a double end lever 66 secured to the end of thevalve plunger 67 for effecting rotation and/or reciprocation thereof.Channels 32 and 33 connected respectively with cylinders 62 and 63 areprovided for introducing pressure fluid to the 35 cylinders to eifectrotation of the value plunger 67 in a clockwise `or counter clockwisedirection as desired. Longitudinal shifting of the valve to effectreversal of the table is effected by introducing pressure\ into thechannel 34 which leads to cylinder 68 formed in bracket 69 attached toone end of the valve body 18 having piston 70 therein engaging one endof valve plunger 67 and by coupling the channel 35 connected to thecylinder 71 formed in the bracket 31 having piston 72 engaging the otherend of valve plunger 67 with the reservoir or vice versa depending uponthe direction of movement desired. To better control the ow of uid inthe channels 32 to 35 inclusive, a pilot valve, such as 36 shown insection in Figure 7, is provided which is also capable of rotary as wellas reciprocatory movement. These movements are effected by a controlplunger 37 adapted to be moved vertically byh dogs, such as 38 and 39,aswell as rotated by dogs 40.or 41 carried on the front edge of thetable. Vertical reciprocation of the plunger 37 by the dogs 38 or 39will effect rotation of the pilot valve shaft 42 thereby chariging therate of movement of the table and rotation of the plunger by either thedogs 40 or 41 will cause reciprocation of the pilot valve and aresultant change of direction in the movement of the table. This isaccomplished in the following manner. Fluid pressure is conducted to 05the pilot valve from the pressure line 24 through branch line 53. 'I'hevalve casing 36'is shown in Figure 7 as' having an inner port connectionas at 73 anda contained distributor bushing 74 having cannelure orgroove 80 circumferentially 7o communicating with the port 'z3 and a.yport 'z5' throughwhich the uid is conducted to the annularv groove 91formed in the valve plunger and thence to the longitudinally extendinghigh pressure bores 78 and 79. Distributor passages 77 75 conduct the nuid to ports 76 formed in annular grooves 75 which communicate with thepipes 45, 33, 32- and 34. Other passages 82 similar to 77 are formed atthe valve plunger which communicate with a pair of longitudinal lowpressure bores 81 angularly displaced 90 with respect to the highpressure bores and discharging into chamber 83 the latterbeing-connected to reservoir by channel 56. 'Thus by proper positioningof the passages 77 and 82, uid pressure may be conducted to selectedchannels while other channels are connected to reservoir.

The selector valve member 67 is of general cylindrical form having thevarious grooves or slots as at 84 for coupling the ports of valve casing18 and intermediate bushing 85 having cannelures 86 and ports 87 alignedwith the ports 88 of casing 18 serving to transmit the power actuatingmedium. Valve `18 may, if desired, be formed with the central passage 89communicating as through outlet 90 with the grooves 84.

It will now be understood that the rotating movement of the selectorvalve will `line up different grooves 84 with different inlet and outletports and that similarly longitudinal sliding of the valve member willalso couple different series of ports through thegrooves according towhether actuating medium is to be directed/into cylinder 13 for urgingthe piston in one direction or the other and for actuating the piston ingiven directions at either a feed ora quick traverse rate.

From the foregoing it should now be apparent that the position of thepilot valve determines the position of the selector valve which in turndetermines the coupling of the pump lines to the table cylinder andthereby the-rate and direction of movement of the table.

Mention has been made that it is desirable at times to cause a pause inthe movement of the work table and usually at a time immediately priorto a reverse movement taking place, whereby the cutter may cut itselffree and thereby properly nish the surface in-contact therewith beforeseparation therefrom. In the present instance the mechanism foraccomplishing this object includes a permanent stop, such as 43, havingadiustably mounted therein av stop` bolt 44 against which a portion ofthe movable table may abut at the end of its forward or feeding stroke.This limits the forward feeding movement ofthe table and since it isdesired to' properly finish the workv before the table returns, meanshave been provided for postponing the shifting of the selector valve. fTo cause the selector valve to move toward the right, as shown in thedrawings, and thereby effect movement of the table to the right,pressure must be admitted to the channel 35. The delay unit isinterposed between the channel 35 and the line 45 leading from the pilotvalve.

This unit comprises a cylinder 46 having reciprocably mounted thereinthe slldable valve 47 having spools 48 and 49 thereon. It will be notedthat when the valve is in the position shown, that the spool 48disconnects the pressure line 45 from the channel 35 thereby preventingthe admission of fluid to effect shifting of the valve to the right. Areduced portion 50 on the valve connects the channel 35 with thereturnline 51 leading to the reservoir. To insure that the valve 47 ismaintained in this 'lowered position pressure is supplied to the uppernd of the cylinder 46 through a channel 52 which is .directly connectedto the channel 34 and adapted to receive pressure from the feed line 24through the branch line 53 leading therefrom `to the pilot valve.Pressure in line 52 will flow through the check valve 53 directly to thetop of the unit.

With the parts in the position shown, Athe table is feeding toward theleft and fluid is flowing 6 in the respective channels as indicated bythe arrows in full lines. In other words, pressure is being admitted tothe right end of the cylinder and fluid is being Withdrawn from thelopposite end by the variable displacement pump 20. At the same timepressure is flowing. from the channel 24 through the branch line 53 ofthe pilot valve which in turn directs it to the channeli34 to the rightend of the selector valve thereby maintaining it shifted toward the leftwhile the channel 35 is connected to reservoir permitting this movement.and the delay unit 46 is maintained in a downward position through theflow of pressure from the channel 52 through the check valve 53'.Simultaneously pressure is being admitted through the channel 32 andwithdrawn through the channel 33 to maintain the selector valve rotatedin the proper position for a feeding movement. Just prior to theengagement of the table with the stop 44 the pilot valve will be dogactuated to the position shown in Figure 3. The channel 45 will now beconnected with the channel 53. Due to the valve 47 being in itslowermost position channel 45 will not be connected to channel 35 whichwill prevent immediate shifting of the selector valve to the right.Since the selector valve is unmoved by this operation the table willcontinue to move to the left and abut the stop 44. However, during' thesmall amount of movement of the table which will take place between thetime of the dog actuation of the pilot valve and the abutting of thestop, the direction of flow in the channels 45 and 52 has been reversedand pressure is being admitted to the'bottom of the valve 46 tending toraise the valve member 47. Since the channel 45 is now connected withthe forward pressure line 24 or in other words with the table propellingpressure lline, the pressure in channel 45 will increase upon abutmentof the table. with a fixed stop. The check valve 53' will closepreventing fiow of fluid therethrough and this will cause the fluid inthe chamber 54 of the delay unit to be forced through the needle valve55.

This valve may comprise a casing having a tapered thread formed thereinfor the reception of a plug having a pipe thread formed on the endlthereof. The flats on the pipe thread permit a.

slight flow of fluid spirally around the plug. It will be apparent thatupon complete tightening of the valve all flow will be prevented while aslight turn of the valve in the opposite direction will permit flow thatmay be varied according to the amount of movement thereof. Fluid flowingthrough the valve will escape to the reservoir through channel 52, thepilot valve and channel 56.

It should now be apparent that the needle valve acts as a throttle todelay the upward movement of the valve member 47 and thus delay theestablishment of pressure connection between the channel 45 and thechannel 35. The time of this delay depends upon the adjustment of thevalve 55. The channel 45 is directly connected with the channel 24 whichinsures positive raising of the valve spool 48. Upon the establishmentof connection between the channel 45 and channel 35, the selector valvewill be immediately shifted to the right, reversing the direction offlow in the channels 16 and 17 causing return movement of the table. Thevalve 47 will remain 7 in an upward position due to the constant pres-"effect reversal is delayed in a manner similar sure in the channel 4 5until the next reversal of the pilot valve. 'Ihe circuit shown in Figurel is arranged to produce a one-way cycle and therefore, upon completionof its return movement, the table will stop. The table may be started,again by manually shifting the pilot valve which will resultin the valve47 being moved down again to its present position.

In reciprocating milling, a work piece is placed upon each end of thetable, and each subjected alternately to the action of the cutter. Ifthe Work were of the type shown in Figure 5, a delay 'in the reversal ofthe table at the end ofv each stroke would be necessary in order toproperly finish the machined surfaces. To this end the circuit shown inFigure l may be modified slightly to effect such a cycle.

In the modification, the admittance of fluid. pressure to the right endof the selector valve to to that previously described. The channel 34 isconnected to the delay valve 46 which now becomes a double acting valve.The channel 45 leading from the pilot valve is divided into two parallelbranches 57 and 58, the branch 57 having the check valve 59 whichpermits flow to the delay unit but prevents return flow and the branch58 having the needle valve 60 through which the return fluid is forcedwhen the check valve 59 closes. i

Upon movement of the table to the right, the pilot valve 36`will be dogactuated to the position shown in Figure 5 connecting the channel 52with pressure thereby urging the valve 47 downward. This downwardmovement is delayed by the clos- 110 ing of the check valve 59 causingthe fluid in the chamber 61 to be forced through the needle valve whichin turn delays shifting of the selector valve.

Completion of the valve movement will establish connection betweenchannels 52 and 34', admit- 115 ting pressure to the right end oftheselector valve. This will shift the selector valve and thus reverse themovement of the table. It will be noted that the valve has now moved toits lower position which is similar to that shown in Figure'l and 120the delayed action in shifting the selector valve to theright will besimilar to that described for that figure.

Thus a delay unit has been provided for a reciprocating cycle circuitwhereby a delayed action is effected at the end of each stroke of thetable. Attention is invited to the fact that the use of separate needlevalves makes it possible to adjust the time of delay independently foreach of the strokes.

That which is claimed is:

1. A machine tool having a reciprocating slide, a fixed stop, hydraulicmeans including a source of pressure for feeding the slide against thestop and returning it to starting position, a reversing valve for saidmeans, hydraulically' actuated means for shifting the valve to effect achange in direction of slide movement, an auxiliary control valve forcoupling said pressure to said hydraulically actuated means, dogactuated means effective upon approach of the slide to the stop to shiftthe control valve to effect a coupling of the pressure for movement ofthe reversing valve, delay means to postpone movement of the reversingvalue, and dog actuated means effective upon return movement of theslide to shift said control valve to directly effect movement of thereversing valve. l

2. A machine tool having 'a reciprocating slide, trip dogs carriedthereby, a fixed stop, hydraulic means including a source of pressurefor feeding the slide against the stop, a reversing valve,-hydraulically actuated means for shifting the valve, a pilot valveactuable by said dogs for determining the direction of iiow to saidhydraulically actuated means, means to delay the ow in one direction topostpone reversing of the slide, and means to by-pass the last namedmeans upon flow in the opposite direction to directly effect reversal ofthe slide.

3. A machine tool having a reciprocating slide, trip dogs carriedthereby, a fixed stop, hydraulic mean including a source of pressure forfeeding the sli e against said stop, reversing valve shiftable to`causeretraction of t e slide from said stop, a fluid operable device forshifting the reversing valve, a pilot valve actuable by said dogs forde'- livering fluid to said iiuid operable device and thereby causeretraction of the slide, a shiftable stopvalve between the pilot valveand said fluid operable device for postponing the pressure ow to saidlast named means, said stop valve having means responsive to thepressure released by the pilot valve to shift the stop valve and effectcoupling after a predetermined interval of time of the pilot valve withsaid iiuid operable device.

4. A machine tool having a reciprocating slide, hydraulic means forreciprocating the same, a reverser for changing the ow and thereby thcdirection of movement of the slide, hydraulically actuated means forshifting the reverser, a source of fluid pressure, control means forconducting uid pressure to said hydraulically actuated means to eectmovement of the reverser in one direction, additional means forconducting pressure to effect movement of the reverser in the otherdirection, and a shiftable delay valve associated with said last namedmeans to stop ow therein, and means in the valve automaticallyresponsive upon admission of pressure thereto to shift the valve slowlyandeffect after a. predetermined interval ofV time the admittance ofpressure to said reverser.

5. A machine tool having a reciprocating slide, hydraulically actuatedmeans for eifecting said reciprocation, a stop, a conduit for admittingpressure to said means to effect movement of the slide against the stop,a pressure shiftable reversing valve in the conduit, a pilot valvetherefor, means effective prior to engagement of the slide with the stopto shift the pilot valve and cause a pressure shifting flow to thereversing valve, a shiftable delay valve positioned to stop saidshifting iiow, means responsive to pressure to shift the delay valve andeffect a delayed opening thereof, a common source of pressure for theslide actuating means and the reversing valve shifting means whereby thesainel pressure utilized for feeding the slide into the stop willactuate the delay valve to shift the reversing valve and thereby causemovement ofthe slide away from the stop.

6. A machine tool having a reciprocating slide, an hydraulic motor foreffecting said reciprocation including a piston and cylinder, one ofwhich is connected to said slide, a source of hydraulic pressure, areversing valve interposed between said source and said cylinder fordetermining the direction of movement of the slide, hydraulic actuatedmeans for shifting said valve, a pilot control valve for said means, anddog control means effective upon movement of the slide in one directionto shift said pilot valve to cause movement of the reversing valve, adelay valve interposed between the pilot valve and the reversing valveto postpone, the admissionbf pressure to the reversing valve, pressureresponsive means in the delay valve for shifting the same to an openposition and means to maintain the delay valve in its shifted openposition upon establishment of pressure connection therethrough to thereversing valve.

7. A machine tool having a reciprocating slide, an hydraulic motor foreffecting said reciprocation, a source of hydraulic pressure, valvecontrol means for determining the effect of the pressure on the motor asto rate and direction, power means for actuating said valve means, tripdogs carried by the slide for controlling the operation of said powermeans and a shiftable delay unit interposed between the power means andvalve means for retarding the actuation thereof, pressure responsivemeans for shifting the delay unit to an inoperative position, said delaymeans being actuated by the forward pressure in the circuit.

8. A machine tool having an hydraulically actuated table, means toreverse the direction of movement of the table including a reversingvalve, power means for shifting said valve, means to delay the shiftingof the valve by the power means including a fluid shiftable valve havingports alternately subjectable to pressure interposed between the powermeans and reversing valve, and a needle valve effective to retardexhaust from one port during admission of pressure to the other of saidports whereby actuation of the shiftable valve and thereby theestablishment of connection between the power means and reversing valvewill be delayed.

9. A milling machine having a support, a table reciprocably mountedthereon-for movement between xed abutments, fluid operable means forreciprocating the table, table control means eifective to change thedirection of movement thereof upon'engagement with each abutmentincluding 115 a fluid operable reversing device, a trip operable pilotvalve shiftable to direct a ilow of actuating uid for power operation ofthe reversing device, spaced means carried by the table for trippingsaid valve as .the table approaches each abutment, means eective toestablish a timed interval delay between shifting of the pilot valve andthe flow from said pilot valve to said fluid operable reversing device,and independent means for adjusting the action of said delay means andthereby the length of the delay at each end of the stroke.

10. A milling machine having a reciprocating table, hydraulic means foreifecting said reciprocation, independent hydraulically actuated meansfor effecting reversal of the table at the end of each stroke, a sourceof pressure, control means for directing said pressure alternatelythrough `independent channels to each of the reversing means, means tointerrupt the flow in either channel and thereby delay reversal of thetable including a shuttle valve shiftable by pressure in either channelto effect opening of that channel and closing of the other, andhydraulic means to retard the shifting movement.

11. A machine tool having a. reciprocating support, power operated meansfor effecting said reciprocation including a reversible motor coupled tothe support, a source of power therefor, reversing means selectivelypositlonable for determining the directional effect of the power onthemotor, 145 power actuable mechanism for positioning said reversingmeans including a control member trip actuable by a control dog carriedby the reciprocating support for potentially coupling the mechanism tothe source of power, a shiftable motion 150 interrupter disconnectingvthe control member from the power actuable mechanism, said interrupterhaving means responsive to the power applied thereto upon said potentialcoupling to effect shifting of the interrupter to a power couplingposition, the time of said shifting movement being sufficient to cause apredetermineddelay between the trip actuation of said control member bythe support and the actual movement of said reversing means 12. Amachine tool having a reciprocating support, power operated means foreffecting said reciprocation including a reversible driving memberoperatively 'coupled to the support, a source of power therefor,reversing means selectively positionable for determining the directionaleffect of the power on'said driving member, power actuable mechanism forpositioning said reversing means including a control member tripactuable by a control dog carried by the support for potentiallycoupling the mechanism to the source of power, a shiftable motioninterrupter disconnecting the control member from the power actuablemechanism, means in the interrupter responsive to the application ofpower resulting from said potential coupling to effect shifting of theinterrupter to' a power coupling position, the time of said shiftingmovement being suilicient to cause a predetermined delay between thetrip actuation of the control member by the support and the shifting ofsaid reversing means.

13. A machine tool having a reciprocating support, power operated meansfor effecting said reciprocation including a reversible motor coupled tothe support, a source of power therefor, reversing means selectivelypositionable for determining the directional effect of the power on thel motor, power actuable lmechanism for positioning said reversing meansincluding a control member trip actuable by the support during movementthereof, a pair of branch lines connecting the coni trol member to saidmechanism, said control member having a rst position for coupling thesource of power to one of said branch lines to effect direct actuationof said mechanism, a second po- .sition for potentially coupling thesource of power to said mechanism, to eifect reverse movement of saidsupport, an interrupter in said last named branch line, means in theinterrupter responsive to the application of power resulting fromsaidpotential'coupling to shift the same to a power coupling position, thetime of said shifting movement being -suicient to cause a predetermineddelay between the trip actuation of saidcontrol member by the supportand the actual movement of said reversing means.

14. In a machine tool having a rotatable cutter and a work support, .thecombination of meansl either position to the other, a pilot valveconstantly connected to said pressure port, delivery channels extendingfrom the pilot valve to the reversing valve actuating pistons, ashiftable valve having a rst port connected to one of said deliverychannels, a second port connected to one of said actuating pistonswhereby coupling of said rst port by the pilot valve to the pump willoperate said shiftable valve to couple said first port to the secondport and thereby a shifting of the reversing valve, said shiftable valvehaving a third port adapted to be connected to reservoir duringoperation thereof, and means to restrict the return flow from said`third port and thereby retard operation of the shiftable valvev andsimultaneously a delay in the shifting of the reversing valve.

15. A delay unitfor an hydraulic control circuit for a machine tool inwhich a reversing valve having a pair of operating pistons determinesthe direction of movement of a movable slide thereof and a dog actuatedpilot valve determines the coupling of pressure to said pistons andthereby power 'shifting of the reversing valve, including a casinghaving a reciprocable plunger therein, a first and second port atopposite ends of the casing, a third port connected to one piston ofthereversing valve and an exhaust port, a rst channel for connecting one ofsaid end ports to the pilot valve whereby upon movement of the pilotvalve to a position to couple pressure to said channel the plunger willbe shifted to a iirst position coupling the third and fourth ports toeffect simultaneous shifting of the reversing valve, and a needle valveoperative to restrict return flow from said fourth port upon admissionof pressure to the first port to eifect shifting `of the plunger in theopposite direction thereby delaying connection of the vfirst port to thethird port and thus a predetermined delay in the shifting of thereversing valve after dog actuation of the pilot valve.

16. A delay unit for an hydraulic control circuit of a machine tool inwhich a reversing valve having a pair of v.operating pistons is employedin the circuit for determining the direction of movement of a movableslide and a dog actuated pilot valve is employed for alternatelycoupling pressure to the reversing valve actuating pistons, including acasing having a shiftable plunger therein, ports at opposite ends of thecasing, a third and fourth port connected to the respective operatingpistons for the reversing valve, an exhaust port intermediate said thirdand fourth ports, a llrst and second channel means connecting therespective end ports to the pilot valve, each of said channels having acheck valve to block return ilow from the ports, and a needle valve inparallel with said check valve operative to retard the return iowwhereby upon coupling of pressure to either of said channel means bythepilot valve, the shiftable valve will be `operated in a delayed mannerto couple pressure to a selected one of said operating pistons to effectshifting of the reversing valve, a predetermined time after dogloperation of the pilot valve.

HANS ERNST. CHARLES W. GOODRICH.

